Order a factory replacement MC for 73-76 power disc brakes and you should be fine. The proportioning valve goes in the line to the rear brakes from the distribution block. I have seen people put it in the line between the MC small reservoir and the distribution block, but that's not how the factory did it. Anyway, here's a tip to save having to cut brake pipes and flare ends: just buy a short (12") length of generic straight pipe from the auto store, and bend it into a loop. Then you can put the proportioning valve at the end of the factory line without cutting it, and run the loop to the distribution block -- it will line right up.
